Most biosensing principles for biochemical markers have been developed for use in in-vitro diagnostics, where a sample is taken (e.g., blood, saliva, urine, mucus, sweat or cerebrospinal fluid) and is transferred to an artificial device (e.g., a plastic disposable) outside a living organism. In such biosensing assays, a wide range of sample pre-treatment steps can be applied (e.g., separation or dilution steps) and multiple reagents can be introduced in the assay (e.g., for target amplification, signal amplification, or washing steps). Examples of in-vitro biosensing assays are: immunoassays, nucleic acid tests, tests for electrolytes and metabolites, electrochemical assays, enzyme activity assays, cell-based assays, etc. (see Tietz, Textbook of Clinical Chemistry and Molecular Diagnostics, 2005).
In in-vivo biochemical sensing, at least a part of the sensor system remains connected to or is inserted in the human body, e.g., on the skin, or in the skin, or below the skin, or on or in or below another part of the body. Due to the contact between the biosensor and the living organism, in-vivo biochemical sensing sets high requirements on biocompatibility (e.g., inflammation processes should be minimized) and the sensor system should operate reliably within the complex environment of the living organism. For monitoring applications, the system should be able to perform more than one measurement over time and the system should be robust and easy to wear.
An important application of in-vivo biochemical sensing is continuous glucose monitoring (CGM). Commercial continuous glucose monitoring devices are based on enzymatic electrochemical sensing (see e.g., Heo and Takeuchi, Adv. Healthcare. Mat. 2013, vol. 2, p. 43-56). Enzymatic sensing is less generic than affinity-based sensing. Commercial systems for in-vivo glucose monitoring are available from Dexcom and Medtronic. A disadvantage of present-day CGM systems is that the sensor response shows drift, and therefore the systems require regular recalibration by an in-vitro blood glucose test (for a review, see e.g., Heo and Takeuchi, Adv. Healthcare. Mat. 2013, vol. 2, p. 43-56).
Several particle-based biosensing techniques are known in the art, including techniques based on detection by optical scattering, e.g., Bruls et al., Lab Chip 2009. Patent application US2012184048 describes a method for the characterization of different bond types in a particle-based biosensing system. Unbound particles bind to a sensing surface in a target-dependent manner, thereafter a bound-free separation is performed (a wash step), and subsequently fluctuations in the intensity of light scattered from the bound particles are measured in order to characterize the bond type, e.g., to discriminate between specifically and non-specifically bound particles. The use of unbound particles and a bound-free separation process is useful for in-vitro diagnostics. However, for in-vivo applications, unbound particles may pose a safety risk, and it is difficult to implement a bound-free separation process in an in-vivo situation.
The above difficulty can be avoided by biosensing techniques based on tethered particle motion (TPM). The TPM technique is based on measurements of the motion of particles tethered to a surface. An example is Laurens et al., Nucleic Acids Res. September 2009; 37(16): 5454-5464, where
TPM experiments are reported on proteins that bind to a DNA tether, in order to reveal how the proteins change the DNA conformation. In such studies, measures are taken to avoid not-via-the-tether binding of the particle to the surface, because a particle that is bound to the surface in another way than via the tether, does not give information about the tether. An example is: Blumberg et al., Biophysical Journal 2005; 89, 1272-1281.
Biosensors having functionalized tethers attached to a surface have been developed based on the principle that the motion of particles attached by a tether changes in dependence upon presence of analyte. The motion changes are due to changes in the structure of the tether itself due to the presence of the analyte. There are also techniques for detecting analytes by measuring a kinematic property of a functionalized particle tethered to a surface in dependence upon presence of analyte. In these techniques, it is important to avoid particle bonding to the surface, because the steric hindrance interferes with sensitivity influenced by the analyte.